1.

0 INTRODUCTION1.1 BACKGROUNDPlasma spray deposition or plasma spraying is a process that combines particle melting, quenching and consolidation in a single operation. The process involves injection of powder particles (metallic, ceramic or cermet powders) into the plasma jet created by heating an inert gas in an electric arc confined within a watercooled nozzle. The temperature at the core of the plasma jet is 10,000-15,000 K. The particles injected into the plasma jet undergo rapid melting and at the same time are accelerated. These molten droplets moving at high velocities (exceeding 100 meters/second) impact on the surface of the substrate forming adherent coating [1,2]. The coating is incrementally built up by impact of successive particles by the process of flattening, cooling and solidification. By virtue of the high cooling rates, typically 105 to 106 K/sec., the resulting microstructures are fine-grained and homogeneous. An overview of plasma spray process including some of the important applications would be given in this article. Plasma spraying has certain unique advantages over other competing surface engineering techniques. By virtue of the high temperature (10,000-15,000K) and high enthalpy available in the thermal plasma jet, any powder, which melts without decomposition or sublimation, can be coated keeping the substrate temperature as low as 500C. The coating process is fast and the thickness can go from a few tens of microns to a few mm. Plasma spraying is extensively used in hi-tech industries like aerospace, nuclear energy as well as conventional industries like textiles, chemicals, plastics and paper mainly as wear resistant coatings in crucial components.

Thermal plasma for plasma spray deposition is generated using plasma spray gun or plasma spray torch. The spray system also includes DC power supplies, cooling water system, gas feeding system, powder feeder and control console. The plasma torch consists of a cathode, made of thoriated tungsten and a nozzle shaped copper anode. Both the electrodes are water-cooled. The electrodes are separated by an insulating block made of nylon that has provision for gas injection. Powder to be spray deposited is injected through an injection port located at the nozzle exit. A DC arc is struck between the anode and the cathode and the arc energy is extracted by the plasma gas, usually argon, which issues out of the nozzle with high temperature (10,000-15,000K) and high velocity. The material can be introduced in the form of wires or rods (Arc wire spraying and rod spraying) or powders, which is the most widely used variant of the process. Metal or ceramic powder is injected into the plasma jet, where the powder particles melt and the molten droplets are accelerated towards the substrate and get deposited. This results in a typical lamellar structure (Fig.1.1). Fig. 1.1 Schematic Diagram Showing Steps for Coating Process

The coating-substrate interface bond mechanism is purely mechanical. Plasma spray deposits typically have lamellar structure with fine-grained microstructure within the lamellae. Atmospheric plasma sprayed coatings also contain varying amounts of retained porosity and inclusions depending on the deposition parameters. There has been a steady growth in the number of applications of thermally sprayed coatings. Availability of hardware and adaptability of the technique are the most important factors for this growth. Plasma spraying has been successfully applied to a wide range of industrial technologies. Automotive industry, aerospace industry, nuclear industry, textile industry, paper industry and iron and steel industry are some of the sectors that have successfully exploited thermal plasma spray technology [2,3]. Plasma spraying has replaced the classical technologies of chrome plating, anodizing and chemical surface hardening of textile machinery parts, such as thread guiding & distribution rollers, tension rollers, thread brake caps, etc, which are in contact with synthetic fibers. For this purpose Al 2O3 + 3% TiO2, Al2O3 + 13% TiO2, and Cr2O3, WC + Co are applied. Detail studies have been carried out on alumina-Titania coating deposition [4,5].The machinery parts with plasma sprayed coatings last 10 to 20 times longer than parts coated by chrome plating or other classical technique [6]. Plasma spraying reduces the idle time of the textile machinery, replacement of worn out parts is minimized, which add to the quality and quantity of textile production, including the life of the textile machinery. In the Paper and printing industry, plasma sprayed coatings of alumina, alumina-titania, or, chromia, are applied on the paper drying rolls, sieves, & filters, roll pins etc. to enhance their lifetime [7,8]. Automotive industries of many industrially advanced countries employ diverse plasma sprayed coatings to improve wear resistance, thermal resistance, resistance to cavitations and corrosion of car parts. It is common to spray

friction surface of steel piston rings with molybdenum or other alloys of type Mo+Chromium carbide + NiCr, Al 2O3+TiO2 [9,10]. One of these materials is selected based on the speed of the engine. Thermal barrier coating (TBC) systems are extensively used in aerospace industry and diesel engines to enhance the life span of critical components. In the aerospace industry, austenitic super alloy blades and vanes of gas turbine engines, combustor cans and turbine shrouds require ceramic coatings to protect them from various degrading forces including hot corrosion, thermal fatigue and oxidation. Experimental and theoretical basis calculations are also studied by Padmanabhan et.al [11] on dependency of operating parameters on ceramic coating deposition. The second area of application is in diesel engines. The primary goals are to insulate components such as pistons, valves, and intake and exhaust ports and to protect moving parts from wear and corrosion. A TBC system, usually, consists of two layers - a metallic bond coat and top ceramic coat. The function of the bond coat is to protect the substrate from oxidation and provide sufficient bonding of the top ceramic coat to the substrate. The insulating ceramic layer provides a reduction of the temperature of the metallic substrate, which leads to improved component durability. Thermal barrier coating system currently employs a duplex design consisting of a bond coat and top ceramic layer. The function of the bond coat is to minimize substrate-coating interface stresses. Bond coat materials that are widely used are MCrAlY(M=Co,Ni) or Pt modified aluminide. Vacuum plasma spraying (VPS) is used for depositing the bond coat. Alternatively, electron beam physical vapor deposition (EB-PVD) can also be used, although the process is more expensive. The standard TBC top layer is YSZ containing 7-8 wt % of yttrium oxide. Since its discovery in 1957, the technology has undergone rapid growth and many innovations have been introduced in the process. In particular, with the developments in VPS and reactive plasma spraying, plasma spray technology has become a versatile Surface Engineering tool with unique processing capabilities.Recently, TiN coatings have been developed using reactive plasma spraying technique [12].

2.0 LITERATURE SURVEY

2.1 PREAMBLEThis chapter deals with survey of literature relevant to the work, namely the development of bond coatings for thermal barrier applications. It embraces on the industrial application of various coating techniques with special reference to plasma spraying, the coating materials and their characteristics. The spheroidal formations of plasma processed powders have also been reviewed.

2.2 SURFACE MODIFICATION: the key to obtain optimum performance

The past decade has seen a rapid development in the range of techniques which are available to modify the surfaces of engineering components. In the last two decades this in turn has led to the emergence to the new field of surface modification. It describes the interdisciplinary activities aimed at tailoring the surface properties of engineering materials. Surface Engineering is the name of the discipline and surface modification is the philosophy behind it. The object of surface engineering is to up grade their functional capabilities keeping the economic factors in mind . It is usually necessary to apply a surface treatment or coating on a base component (substrate) in order to design a composite system, which has a performance, which cannot be achieved by either of the base component or the surface layer alone . Thus, through a surface modification process, we assemble two (or more) materials by the appropriate method and exploit the qualities of both .

Cutting tools : Cutting tools are subjected to a high degree of abrasion.WC-Co

composite is a very popular cutting tool material, and is well known for his high hardness and wear resistance. If a thin coating of TiN (CVD) is applied on to the WCCo insert, its capability increases considerably [6]. TiN is more capable of combating abrasion. On the other hand, TiN is extremely brittle, but the relatively tough core of WC-Co composite protects it from fracture. Surface modification is a versatile tool for technological development provided it is applied judiciously keeping in mind the following issues. The coating-surface treatment should not impair the properties of the bulk material. The choice of technique must be capable of coating the component, in terms of both size and shape.

The technological value addition should justify the cost. A suitable classification system for surface modification is given in.

It can be seen that the advanced technologies can be subdivided into gaseous, molten or semi-molten state processes , which are dominated by Wet Process (Traditional) Dry Process (Advanced) Liquid Phase Sol-Gel Electro- deposition Gaseous Phase Molten, Semi-liquid Phase PVDCVDII Thermal spraying Laser Welding dry methods. These dry coating methods are recognized as having less environmental impact than the traditional wet processes such as electroplating and salt bath nitriding.Of all advanced coatings techniques, thermal spraying has gradually emerged as the one of the most industrially useful and versatile because of the wide range of coating materials and substrates that can be processed, ranging from gas turbine technology (heat engines) to the electronics industry (tape recording heads). Thermal spraying has been practiced since the early part of 20th century when the first oxy-acetylene torches were modified to deposit powders and wires . Processes available for thermal spraying have been developed specifically for a purpose and fall into two categories-high and low energy processes.

2.3 THERMAL PLASMA GENERATION

Classification of Plasma: Plasmas of technological interest can be classified into two categories, namely, Non-equilibrium plasmas and Thermal plasmas .Non-equilibrium plasma or cold plasmas, more popularly known as glow-discharge plasmas, are lowpressure plasmas characterized by high electron temperatures ( Te ) and low ion and neutral particle temperatures ( Ti ). They are widely used in lighting, surface cleaning, etching, film deposition and polymerization. Thermal plasmas or hot plasmas are characterized by the electron temperature being approximately equal to the gas temperature ( Tg )and the plasma is said to be in local thermal equilibrium. Normally, plasmas in the temperature range of 2,000 30,000 K and

with charged particle density of 1019-1021 m-3 are termed thermal plasmas. Thermal plasma processing has been successfully applied to develop advanced ceramic coatings, synthesis of nanocrystalline materials, processing of minerals and ores, and treatment of hazardous wastes. Plasma retains many of the properties of gases, and behaves in conformance with the physical laws valid for the gases. The specific properties of plasma that distinguish it from a gas become apparent in the presence of a strong magnetic field, when the plasma acquires non-isotropic property. Thus, the fundamental difference between plasma and non-ionized gas lies in their response to electro-magnetic forces. The electrically charged particles present in a plasma are effected by externally applied magnetic and electric fields and also interact with one another .The electric field which they setup is so extensive that every particle is effected by a multitude of other particle.Consequently, the energy bonds between the particles are much stronger in plasma than in a non-ionized gas.

DC PLASMA TOURCH : The design of a typical DC plasma torch is based on a rod

type cathode and nozzle shaped anode (both are strongly water cooled) with tangential gas entry through the insulator module. When a gas is injected into the electrode gap and a high intensity current is passed, a DC arc is established between the electrodes. The plasma gas extracts energy from the arc and emerges out of the nozzle (due to forced flow of gas) as a high temperature, high velocity jet. The temperature at the core of the plasma jet ranges between 15,000 K and 20,000 K. A thermal pinch effect is produced by the joint action of the cold wall arc channel and the cold gas sheath around a very high-temperature conducting core (the arc column).Improper gas flow may lead to blowing out of the flame or fail to create the necessary thermal pinch effect to force the arc down the nozzle.

The DC arc in a plasma torch needs to be stabilized, i.e. it should be remain stationary against fluctuation. This is often done by constricting the arc to a welladjusted narrow high-temperature, highly conducting arc column. Various torch configurations are possible depending upon the stabilization mode: tangential vortex gas input in the arc channel, axial gas input along the cathode, segmented anode arc and magnetic stabilization [10]. The magnetic field can be self-induced (by an arc current greater than 8000 A), or externally generated. The electric arc is stabilized by using a constricted anode nozzle and by the resultant aerodynamic effects in the streaming plasma gas. Stabilizing action of the vortex gas flow provides a cold boundary layer near the anode wall so that heat loss to the wall is reduced. This results in the thermal energy being highly concentrated with improved torch stability and efficiency. Electrodes (cathode and anode) are chosen depending upon the desired performance for a particular application. The material for electrodes may be consumable (graphite) or non-consumable (copper, tungsten or molybdenum). The obvious choice of material for the anode is copper although molybdenum and graphite are also used. The cathode can be of thermionic type such as tungsten, carbon or molybdenum, which obviously, must be used in nonoxidizing atmosphere. Under certain conditions of oxidizing atmosphere, one can use zirconium or hafnium cathodes. Non-thermionic cathodes are normally made of copper. The cathodes are usually made of tungsten enriched by 2% ThO2.Thorium in tungsten serves predominantly for lowering the electron emission potential and hindering of cathode wear due to impurities in the plasma gas .Depending upon the nature of the gas and the working parameters, the anode losses range between 8 and 10 % of the energy input in the arc. The anode losses are proportional to the current density and are a function of the arc voltage. Heat losses at the cathode are generally quite low (less than 10% of the input power) .The shape of the electrodes is another aspect depending upon the application for the plasma torch. Anodes are usually in the shape of tubes, disks, nozzles or rings. Nozzle shaped anodes are the standard design, where they serve as both an electrode and an arc constrictor (increasing the enthalpy level of the emanating plasma jet).The shape of the cathode is mainly determined by the range of operating arc current. Cathodes consisting of a rod with either a sharp pointed or conical tip; are useful for currents up to about 1,000 amperes and button type cathodes may be useful up to around 5,000 amperes. Extending electrode life time is a very important aspect of thermal plasma research. In order to reduce electrode wear electrodes are protected by efficient cooling systems. Electrode erosion is reduced by rapid motion of the arc attachment by means of gas flow or magnetic fields. Cathode erosion can be a severe problem if reactive gases are used in the arc.

Advantages for Thermal Plasma Torches: As a heat source the major advantages are: a. A very concentrated enthalpy and the resultant high temperature, much greater than achievable with fossil fuels, b. A clean and adjustable reaction atmosphere (reducing, oxidizing or inert), c. Smaller amounts of off gas to recycle or treat, d. Capable of small portable reactor design.

3.0 PRINCIPLEPLASMA SPRAYING AND PROCESS CONDITIONSPlasma spraying is a material processing technique which uses the energy of an electric arc and gases to generate a plasma beam capable of melting and depositing metallic and non-metallic materials on a substrate. This technique has been used to develop protective coatings of ceramics, alloys, and composites to enhance the surface properties of critical components operating in severe environment. In conventional plasma spraying, an arc is created between a rod/stick type throated tungsten cathode and a nozzle type copper anode (both water cooled). Plasma generating gas is forced to pass through the annular space between the electrodes. While passing through the arc, the gas undergoes dissociation and/or ionization in the high temperature environment resulting plasma. The ionization is achieved by collision of electrons of the arc with the neutral molecules of the gas. The plasma protrudes out of the electrode encasement in the form of a jet. The material to be coated is introduced into the plasma jet in powder form in metered quantity by means of a carrier gas. The powder particles, as they enter the plasma jet, are heated and melted and the

molten droplets absorb the momentum of the expanding gas and are accelerated to a very high velocities (exceeding 100 m/s).As these molten droplets strike the substrate surface, they flatten and get anchored to the surface irregularities to form an adherent coating. The coating builds up layer by layer. Plasma spraying has certain unique advantages over other competing surface engineering techniques. By virtue of the high temperature (10,000-15,000K) and high enthalpy available in the thermal plasma jet, any powder, which melts without decomposition or sublimation, can be coated keeping the substrate temperature as low as 500C. The coating process is fast and the thickness can go from a few tens of microns to a few mm. The spraying technique does not impose any restriction on the work piece dimensions and large samples can be coated. In plasma spraying one has to deal with a lot of process parameters. An elaborate listing of these parameters and their effects are reported in the literature. Some important process parameters and their roles are listed below. Roughness of the substrate surface Cleanliness of the substrate Cooling water Arc power Plasma gas Carrier gas Mass flow rate of powder Standoff distance (TBD) Spraying angle Substrate cooling Powder related variables Preheating of the substrate Angle of powder injection

Roughness of the substrate surface : A rough surface provides a good

coating adhesion. A rough surface provides enough room for anchorage of the splats facilitating bonding through mechanical interlocking. A rough surface is generally created by shot blasting technique. The shorts are kept inside a hopper, and compressed air is supplied at the bottom of the hopper. The shorts are taken afloat by the compressed air stream into a hose and ultimately directed to an object kept in front of the exit nozzle of the hose. The shorts used for this purpose are irregular in shape, highly angular in nature, and made up of hard material like alumina, silicon carbide, etc. Upon impact they create small craters on the surface by localized plastic deformation, and finally yield a very rough and highly worked surface. The roughness obtained is determined by shot blasting parameters, i.e., shot size, shape and material, air pressure, standoff distance between nozzle and the job, angle of impact, substrate material etc. The effect of shot blasting parameters on the adhesion of plasma sprayed alumina has been studied. Mild steel serves as the substrate material. The adhesion increases proportionally with surface roughness and the parameters listed above are of importance. A significant time lapse between shot blasting and plasma spraying causes a marked decrease in bond strength.

Cleanliness of the substrates : The substrate to be sprayed on must be free

from any dirt or grease or any other material that might prevent intimate contact of the splat and the substrate. For this purpose the substrate must be thoroughly cleaned (ultrasonically, if possible) with a solvent before spraying. Spraying must be conducted immediately after shot blasting and cleaning. Otherwise on the nascent surfaces, oxide layers tend to grow quickly and moisture may also affect the surface. These factors deteriorate the coating quality drastically.

Cooling water : For cooling purpose distilled water should be used, wheneverpossible. Normally a small volume of distilled water is recirculated into the gun and it is cooled by an external water supply from a large tank. Sometime water from a large external tank is pumped directly into the gun [27].

Arc power : It is the electrical power drawn by the arc. The power is injected in tothe plasma gas, which in turn heats the plasma stream. Part of the power is dissipated as radiation and also by the gun cooling water. Arc power determines the mass flow rate of a given powder that can be effectively melted by the arc. Deposition efficiency improves to a certain extent with an increase in arc power, since it is associated with an enhanced particle melting. However, increasing power beyond a certain limit may not cause a significant improvement. On the contrary, once a complete particle melting is achieved, a higher gas temperature may prove to be harmful. In the case of steel, at some point vaporization may take place lowering the deposition efficiency.

Plasma gas : The most commonly used gases for plasma generation are argon,nitrogen, helium, hydrogen and air. Plasma gas flow rate and the electric power to the plasma torch must be properly balanced in order to get a stable arc. The choice of plasma gas depends on many factors, such as the design features of the torch, in particular the electrode materials .In the case of plasma torches employing tungsten cathode, the choice of plasma gas is limited to inert gases and nonoxidizing gases. Gas enthalpy is another important factor deciding the choice of the gas. The major constituent of the gas mixture is known as primary gas and the minor is known as the secondary gas. The neutral molecules are subjected to the electron bombardment resulting in their ionization. Both temperature and enthalpy of the gas increase as it absorbs energy. Since nitrogen and hydrogen are diatomic gases, they first undergo dissociation followed by ionization. Thus they need higher energy input to enter the plasma state. This extra energy increases the enthalpy of the plasma. The heat transfer coefficient is also higher, enabling efficient plasmaparticle heat transfer. Movement of molecules Collision Exchange energy & pulse Velocity increases Frequency of collision increases Temperature increases Ionization Dissociation Ionization Di-atomic gas Mono-atomic gas Energy. On the other hand, the mono-atomic plasma gases, i.e. argon or helium, approach a much higher temperature in the normal enthalpy range. Good heating ability is expected from them for such high temperature. In addition, hydrogen followed by helium has a very high specific heat, and therefore is capable of acquiring very high enthalpy. When argon is doped with helium the spray cone becomes quite narrow which is especially useful for spraying on small targets. The low cost and high internal energy of nitrogen makes it the most commonly used gas. If a completely inert atmosphere is required, argon is usually preferred. Reactive gases like hydrogen, oxygen, chlorine, methane and ammonia/nitrogen can be used to impart reducing, oxidizing, chloriding, carburizing, or nitrating effects, respectively, to the plasma.

Carrier gas : Particle injection into the plasma stream is not a trivial problem. Infact, it can be very difficult due to the high viscosity of the plasma. Precursor powders are usually entrained in a carrier gas for injection into the plasma. The carrier gas can be inert or reactive. Normally the primary gas itself is used as a carrier gas. The flow rate of the career gas is an important factor. A very low flow rate cannot convey the power effectively to the plasma jet, and if the flow rate is very high then the powders might escape the hottest region of the jet. There is an optimum flow rate for each powder at which the fraction of unmelted powder is minimum and hence the deposition efficiency is maximum.

Mass flow rate of powder : Ideal mass flow rate for each powder has to bedetermined. Spraying with a lower mass flow rate keeping all other conditions constant results in under utilization and slow coating buildup. On the other hand, a very high mass flow rate may give rise to an incomplete melting, resulting a high amount of porosity in the coating. The unmelted powders may bounce off from the substrate surface as well keeping the deposition efficiency low.

Torch to base distance : It is the distance between the tip of the gun and thesubstrate surface. A long distance may result in freezing of the melted particles before they reach the target, whereas a short standoff distance may not provide sufficient time for the particles in flight to melt. The relationship between the coating properties and spray parameters in spraying alpha alumina has been studied in details. It is found that the porosity increases and the thickness of the coating (hence deposition efficiency) decreases with an increase in standoff

distance. The usual alpha-phase to gamma-phase transformation during plasma spraying of alumina has also been restricted by increasing this distance. A larger fraction of the unmelted particles go in the coating owing to an increase in torch to base distance.

Spraying angle : This parameter is varied to accommodate the shape of the

substrate. In coating alumina on mild steel substrate, the coating porosity is found to increase as the spraying angle is increased from 300 to 600. Beyond 600 the porosity level remains unaffected by a further increase in spraying angle .The spraying angle also affects the adhesive strength of the coating .The influence of spraying angle on the cohesive strength of chromia, zirconia-8 wt% yttrium, and molybdenum has been investigated, and it has been found that the spraying angle does not have much influence on the cohesive strength of the coatings.

Substrate cooling : During a continuous spraying, the substrate might get heatedup and may develop thermal-stress related distortion accompanied by a coating peel-off. This is especially true in situations where thick deposits are to be applied. To harness the substrate temperature, it is kept cool by an auxiliary air supply system. In additions, the cooling air jet removes the unmelted particles from the coated surface and helps to reduce the porosity.

Powder related variables : These variables are powder shape, size and sizedistribution, processing history, phase composition etc. They constitute a set of extremely important parameters. For example, in a given situation if the powder size is too small it might get vaporized. On the other hand a very large particle may not melt substantially and therefore will not deposit. The shape of the powder is also quite important. A spherical powder will not have the same characteristics as the angular ones, and hence both could not be sprayed using the same set of parameters.

Preheating of the substrate : The nascent shot blasted surface of the substrateabsorbs water and oxygen immediately after shot blasting. Before spraying, the substrate should be preheated to remove moisture from the surface and also for a sputter cleaning effect of the surface by the ions of the plasma.

Angle of powder injection : Powders can be injected into the plasma jetperpendicularly, coaxially, or obliquely. The residence time of the powders in the plasma jet will vary with the angle of injection for a given carrier gas flow rate. The residence time in turn will influence the degree of melting of a given powder. For example, to melt high melting point materials a long residence time and hence oblique injection may prove to be useful. The angle of injection is found to influence the cohesive and adhesive strength of the coatings as well.

4.0 INDUSTRIAL APPLICATIONS OF PLASMA SPRAYING

Plasma spraying is extensively used in hi-tech industries like aerospace, nuclear energy as well as conventional industries like textiles, chemicals, plastics and paper mainly as wear resistant coatings in crucial components. There has been a steady growth in the number of applications of thermally sprayed coatings. Availability of hardware and adaptability of the technique are the most important factors for this growth. Plasma spraying has been successfully applied to a wide range of industrial technologies. Automotive industry, aerospace industry, nuclear industry, textile industry, paper industry and iron and steel industry are some of the sectors that have successfully exploited thermal plasma spray technology [5, 15]. Textile Industry: Plasma spraying was for the first time employed in textile industry in Czechoslovakia. Plasma spraying has replaced the classical technologies of chrome plating, anodization and chemical surface hardening. Advantages of this technique are a lot, all of which add to the quality and quantity of textile production. Critical machinery parts: Different thread guiding & distribution rollers, ridge thread brakes, distribution plates, driving & driven rollers,gallets, tension rollers, thread brake caps, lead-in bars etc. Coatings and advantages : High wear resistance coatings are required on textile machinery parts which are in contact with synthetic fibers. For this purpose especially Al 2O3 + 3% TiO2, Al2O3 + 13% TiO2, Cr2O3, WC + Co are applied. These coatings with hardness ranging from 1800 to 2600 HRV are extraordinarily dense, have high wear resistance and provide excellent bonding with the substrate. Plasma spraying has following advantages in textile industries: Replacement of worn out parts is minimized and hence reduces the idle times Physical and mechanical properties of fibers are improved Revolution speed of these lighter parts can be increased

Shelf life of the textile machinery parts with plasma sprayed coating last 5 to 20 times longer than parts coated by chrome plating or another classical technique Economic savings are realized considerably by substituting heavy steel or cast iron parts with aluminum or durable ones with wear- resistant coatings

Paper and printing industry: The machinery in the paper and printing industry is usually quite large and is subjected to considerable wear from the sliding and friction contact with the paper products. Critical machinery parts: Paper drying rolls, sieves, filters, roll pins etc.in paper machines, printing rolls, tension rolls and other parts of printing machines. Coatings and advantages: Spraying of oxide layers is an available economical solution which can be employed right in place in the production shop. Here again oxide layers composed of Al2O3 with 3 to 13 % additions of TiO2, Cr2O3 or MnO2 are applied. Cast iron rolls are typically first sprayed with NiCr 80/20, 50m thick and then over it 0.2mm thick Al2O3 + 13% TiO2 layer is coated. The special advantages are mentioned below: Ensures corrosion resistance of rolls i.e. the base metal Resistance of oxide layers against printing inks extends the life of machine parts Production cost is reduced considerably Coating resulted to the so-called orange peel phenomena, surface finishing obtainable that prevents paper foil, dyes etc.from sticking and allows their proper stretching Automotive Industry and the production of Combustion engines: Plasma sprayed coatings used, in automotive industries of many industrially advanced countries, endure higher working pressure and temperature to improve wear resistance, good friction properties, resistance against burn-off and corrosion due to hot combustion products and resistance against thermal loading. Some of the several applications developed for the automotive industry at the Slovak Academy of Sciences (SAV) in Bratislava are spraying torsion bars with aluminium coatings against corrosion. The plasma spraying technology is introduced in the production of gear-shift forks for gear boxes in fiat car factory and on the critical parts of big

Diesel engines. The coating materials and their advantages are given below, Table 2.3. Glass Industry: Molten glass quickly wears the surface of metal which comes in contact with it. In order to protect the metal tools, plasma sprayed coatings are made onto it. The machine parts, typical coatings used and their advantages are tabulated below, Table 2.4.

Electrochemical Industry: In the electromechanical and computer industries the electrically conductive Cu, Al, W and the semi-conductive and insulating ceramic layers are widely used. Some contacts of electrodes, e.g. the spark gaps of nuclear research equipment, are produced of massive tungsten. Such electrodes can be replaced by modern electrodes with a sprayed tungsten coating about 0.5mm thick. This electrode ensures short- time passages of 300,000A current with a life of several hundred switching. Some more applications are given below, Table 2.5. Hydraulic machines and mechanisms: The range of possible applications in this field is very extensive, mainly in water power plants, in production and work of pumps, where many parts are subjected to combined effects of wear, corrosion, erosion and cavitations. Rolling mills and foundry: In Rolling mills and pressing shops the wear resistant coatings are used to renovate the heavy parts of heavy duty machines whose replacement would be very costly. Several applications in this field are presented herewith: Rolling strand journals being repaired by giving a coating layer of stainless steel. Blooming roll mill journal renovated with a NiCrBSi layer. Gears of rolling mill gear box being renovated by a wear resistance coating. To repair a rolling mill slide and the plungers of a forging press a hard wear resistance is applied. Heat resistant plasma coating is widely used for foundry and metallurgical equipment where molten metal or very high temperatures are encountered. This equipment includes the sliding plugs of steel ladles with alumina or zirconia coatings. Conveyer rollers in plate production with zirconia based refractory coatings, Oxygen tubes, cast iron moulds in continuous casting of metals, with Al2O3+TiO2,ZrSiO4+ZrO2+MgO

High Temperature wears resistance coatings on Slide Gate Plates: In steel plants severe erosion of refractory teeming plates (slide gate plates) and generation of macro-micro cracks during teeming of steel is observed, rendering the plates unstable for reuse. Plasma sprayed ceramic coatings on refractory plates is made to minimize the damage and hence increase the life of slide gate plate.Al 2O3, MgZrO3, ZrO2, TiO2, Y2O3 and calcia stabilized. Zirconia can be coated.

Chemical Plants: The base metal of machine parts is subjected to different kind of wear in chemical plants. In such cases plasma sprayed coatings are applied to protect the base metal. They can be used for various blades, shafts, bearing surfaces, tubes, burners, parts of cooling equipments etc.Few specific applications are tabulated below, Table 2.7. Critical parts Blades of a chemical mixer Roll for the production of plastic foils

Typical coatings

Advantages

NiCrBSi Al2O3

Increases wear resistance of surfaces. Increases wear resistance of surfaces and keep the foil from adhering to the surface. Increases resistance against abrasion and

Fan blades

aggressive vapors Polymer Cutter Nozzle worn by rotary friction movement during the production of granulated polymer Induction Flow meter Cutter nozzle sprayed with WC+ 12% Co deposited on the annulus ZrSiO4 on the internal surfaces WC having the property of hard, tough and wear resistance prolongs the life of equipment. Provide resistance to wear, hot and corrosion of aggressive fluids like NH4NO3,NH4OH and the meter functions properly forming a dielectric layer

5.0 PLASMA SPHEROIDIZATION

Powders of metals, alloys and ceramics can be melted in a plasma jet. Melting of particles results in the formation of a spherical drop under the action of the surface tension forces and this shape is usually retained after solidification, thus providing the name to the process. This treatment may be used simply to give a spherical particle shape for particular application, such as Fe 3O4 for photocopying, plasma spray quality powders for thermal spray applications and UO 2 for dispensed nuclear fuels. Spheroidization is particularly useful to prepare spray quality powders of special materials for thermal spray applications [147]. Spheroidization of a powder blend of Ni-15%Al has been carried out in a thermal plasma reactor [148]. The as-collected powder showed poor crystallinity as indicated by broad X-ray diffraction pattern [149]. Annealing at 800K in an atmosphere of flowing argon resulted in the formation of well-crystalline Ni 3Al. Besides possessing spherical morphology, the powder has excellent flow characteristics, making it ideally suited for thermal spray application.

Spheroidized particles may also be prepared by atomization of rods or wires fed into a plasma. The diameter of the particles produced depends on the diameter of the torch nozzle, the gas flow rate, plasma density and temperature and surface tension of the material in the liquid state [150]. Alumina particles of 40 m diameter have been produced by feeding alumina rod in hydrogen plasma. The heat transfer can be improved if the arc is transferred to the wire but this is not possible with nonconducting materials. An example of large scale industrial application is the spheroidization of magnetite for photocopying applications, where magnetite particles, 125 m diameter, are produced in 600 kW AC air plasma heater with a power consumption of 2 kWh/kg [151]. Powder plasma treatment rounds up the particulates and refined them from contaminations. For example, by spheroidization of the aluminium oxide powder the content of other oxides (Na, Fe, and Mg) is diminished. The refining effect of plasma powder processing, simultaneously spheroidizing particulates, helps to create new porous materials for various purposes (filters, cathodes, electrodes) to operate at very high temperatures. Spherical particulates of aluminium oxide are used to produce new types of cathodes, powerful vacuum tubes. Spherical refractory powders, obtained in plasma provided a means for developing new refractory products. When manufacturing powder materials in plasma jets, the dispersity and shape of powder particles, their purity and surface physico-chemical properties are controlled by the jet parameters (power, temperature, flow rate, gas partial pressure) and the tempering intensity. As the reactions proceed within the plasma jet and its wake, the processed materials have no contact with the reactor walls, so the reaction products are not contaminated by the lining material.Therefore,a jet of plasma makes it possible to obtain high purity powders (ultra dispersed, spheroidized, composite etc.) based on metals, alloys, oxides, nitrides, carbides, hydrides and complex compounds.

6.0 PROBLEMS FOUND IN PLASMA APPLICATION AND ITS POSSIBLE SOLUTIONS

1. WATER INLET TEMPERATURE IS TOO HIGH DUE TOCAUSES: .the supply voltage is missing

.the use of defective thermostat . defects in water cooler i.e. when the cooling capacity is insufficient

REMEDIES: It can be prevented by: .checking the supply voltage before starting the operation .checking the thermostat and replacing it if necessary .checking the water cooler before starting

2. WATER INLET TEMPERATURE IS LOW:

CAUSES: .the supply voltage is missing . the use of defective thermostat or conductance gauge . defects in water cooler i.e. when the cooling capacity is insufficient

REMIEDIES: It can be prevented by: .checking the supply voltage before starting the operation .checking the thermostat and replacing it if necessary .checking the water cooler before starting

3. WATER OUTLET TEMPERATURE IS HIGH:

CAUSES: .the supply voltage is missing .the use of defective thermostat . defects in water cooler i.e. when the cooling capacity is insufficient .when the performance of spray gun is too high

REMIEDIES:

It can be prevented by: .checking the supply voltage before starting the operation .checking the thermostat and replacing it if necessary .checking the water cooler before starting .checking the parameter of spray gun before starting the operation

4. WATER CONDUCTIVITY IS HIGH

CAUSES: .the poor cooling water quality .due to the defects in conductance sensor or in measuring device which deviates the upper limit for cooling water conductivity

REMIEDIES: .if the cooling water is found to be poor i.e. its conductivity is low then the conductivity is exchange .checking the parameters of conductance sensor or measuring device before starting the operation

5. WATER OUTLET TEMPERATURE IS LOW:

CAUSES: . the use of defective thermostat . defects in water cooler i.e. when the cooling capacity is insufficient .due to the missing supply voltage the lower limit outlet temperature cooling water is violated

REMIEDIES: It can be prevented by: .checking the supply voltage before starting the operation .checking the thermostat and replacing it if necessary .checking the water cooler before starting

6. Water flow is high.

Causes: due to the defects in conductance sensor or in measuring device which deviates the upper limit for cooling water conductivity . the use of defective thermostat . defects in water cooler i.e. when the cooling capacity is insufficient

REMEDIES: It can be prevented by: .checking the supply voltage before starting the operation .checking the thermostat and replacing it if necessary .checking the water cooler before starting

7. IGNITION FAILED:CAUSES: Ignition did not take place because supply voltage is missing. Defective ignition relay, ignition device or fuse. Insufficient isolation for spray gun.

REMEDIES: Checking the isolation for spray gun. Checking the supply voltage before starting the operation. Check and replace ignition relay or ignition unit if necessary.

8. COOLING WATER:CAUSES: The cooling water flow is 4 lit/min and the flow meter is triggered. Water cooler switched off or hose burst. Pump defective and too little water in water cooler.

REMEDIES: Check cooling circuit of unicoated system.

Check water cooler. Check function of flow meter.

9. EXHAUST FLOW SWITCH

10. NO FLOW AIR SENSOR

CAUSES: There is no flow air sensor when the exhaust unit for spray cell is not working or due to the defects in fan, fuses e.g. or the suction channel is blocked or flow monitor is defective. REMEDIES: Checking fan and fuse. Checking function and flow monitor.

11. NO WATERCAUSES: The cooling water flow is 4 lit/min and the flow meter are triggered. Water cooler switched off or hose burst. Pump defective and too little water in water cooler.